The invention relates to a frequency demodulator comprising a first gate circuit having an "exclusive-OR" function and receiving at its two inputs square-wave signals having the frequency of the frequency-modulated signal, delay means for delaying one of said square-wave signals relative to the other, and a low-pass filter connected to the output of the first gate circuit.
A frequency demodulator is, for example, used to recover data transmitted by frequency shift keying a carrier.
A frequency demodulator of the above-defined type is disclosed in, for example, U.S. Pat. No. 3,778,727. In prior art demodulators of this type, a delay circuit is employed to produce a fixed delay of one of the square-wave signals applied to the "exclusive-OR" circuit. This fixed delay is equivalent to a phase shift of the delayed signal proportional to the frequency. A pulse-shaped signal formed by pulses appearing at the transition instants of the non-delayed signal and having a fixed duration is obtained at the output of the "exclusive-OR" circuit. The low-pass filter connected to the output of the "exclusive-OR" circuit supplies the average value of this pulse-shaped signal, which average value is proportional to the phase shift between the delayed and non-delayed signals and consequently proportional to the frequency of the modulated signal. The fixed delay can be realized by means of a shift register in accordance with the technique employed in the above-mentioned U.S. Pat. No. 3,778,727 or by means of any type of analog delay circuits.
Such a demodulator has a response which is basically linear if the delay .tau. is independent of the frequency F of the modulated signal, which however is not always easy to realize by means of delay circuits of an analog type. However, in view of the fact that its response is of necessity zero for F=0 and at its maximum for F=1/2.tau., it has the drawback that it only produces a relatively weak variation of its output signal as the frequency range .DELTA.F of the modulated signal is relatively narrow and far remote from the frequency F=0.